MIPS: initial stack protector support

Implements basic stack protector support based on ARM version in
c743f38013aeff58ef6252601e397b5ba281c633 , with Kconfig option,
constant canary value set at boot time, and script to check if
compiler actually supports stack protector.

Tested by creating a kernel module that writes past end of char[].

Signed-off-by: Gregory Fong <[e-mail]>
Cc: [e-mail]
Cc: Filippo Arcidiacono <[e-mail]>
Cc: Carmelo Amoroso <[e-mail]>
Patchwork: https://patchwork.linux-mips.org/patch/5448/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/Kconfig

···
 
 	  If unsure, say Y. Only embedded should say N here.
 
+config CC_STACKPROTECTOR
+	bool "Enable -fstack-protector buffer overflow detection (EXPERIMENTAL)"
+	help
+	  This option turns on the -fstack-protector GCC feature. This
+	  feature puts, at the beginning of functions, a canary value on
+	  the stack just before the return address, and validates
+	  the value just before actually returning.  Stack based buffer
+	  overflows (that need to overwrite this return address) now also
+	  overwrite the canary, which gets detected and the attack is then
+	  neutralized via a kernel panic.
+
+	  This feature requires gcc version 4.2 or above.
+
 config USE_OF
 	bool
 	select OF

arch/mips/Makefile

···
 
 LDFLAGS			+= -m $(ld-emul)
 
+ifdef CONFIG_CC_STACKPROTECTOR
+  KBUILD_CFLAGS += -fstack-protector
+endif
+
 ifdef CONFIG_MIPS
 CHECKFLAGS += $(shell $(CC) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
 	egrep -vw '__GNUC_(|MINOR_|PATCHLEVEL_)_' | \

arch/mips/include/asm/stackprotector.h

···
+/*
+ * GCC stack protector support.
+ *
+ * (This is directly adopted from the ARM implementation)
+ *
+ * Stack protector works by putting predefined pattern at the start of
+ * the stack frame and verifying that it hasn't been overwritten when
+ * returning from the function.  The pattern is called stack canary
+ * and gcc expects it to be defined by a global variable called
+ * "__stack_chk_guard" on MIPS.  This unfortunately means that on SMP
+ * we cannot have a different canary value per task.
+ */
+
+#ifndef _ASM_STACKPROTECTOR_H
+#define _ASM_STACKPROTECTOR_H 1
+
+#include <linux/random.h>
+#include <linux/version.h>
+
+extern unsigned long __stack_chk_guard;
+
+/*
+ * Initialize the stackprotector canary value.
+ *
+ * NOTE: this must only be called from functions that never return,
+ * and it must always be inlined.
+ */
+static __always_inline void boot_init_stack_canary(void)
+{
+	unsigned long canary;
+
+	/* Try to get a semi random initial value. */
+	get_random_bytes(&canary, sizeof(canary));
+	canary ^= LINUX_VERSION_CODE;
+
+	current->stack_canary = canary;
+	__stack_chk_guard = current->stack_canary;
+}
+
+#endif	/* _ASM_STACKPROTECTOR_H */

arch/mips/kernel/process.c

···
 	return 1;
 }
 
+#ifdef CONFIG_CC_STACKPROTECTOR
+#include <linux/stackprotector.h>
+unsigned long __stack_chk_guard __read_mostly;
+EXPORT_SYMBOL(__stack_chk_guard);
+#endif
+
 /*
  *
  */